The present invention relates to a time-division multiplex communication system.
FIG. 1 shows one time-division multiplex communication system disclosed in Japanese patent application No. 1990-153016. The disclosed time-division multiplex communication system comprises a single master station B, a plurality of repeater stations R, and a plurality of slave stations T, these stations B, R, T being connected in a hierarchical network. The master station B and any one of the slave stations T communicate with each other when a down frame signal indicating the station number of the slave station T is transmitted from the master station B to the slave station T directly or through a repeater station or stations R, and an up burst signal is transmitted from the slave station T, which is permitted to transmit the signal, to the master station B directly or through the repeater station or stations R.
In the time-division multiplex communication system, to prevent up burst signals transmitted by plural repeater stations R and plural slave stations T from overlapping in time, the up burst signals are adjusted with respect to delay times by the repeater stations R and the slave stations T, as follows: To compensate for the difference between radio wave delays due to the distance between the stations, the up burst signal are delayed depending on the distance between the stations. To compensate for the difference between radio wave delays due to the difference between the numbers of layers or levels from the master station B, the up burst signals are delayed by certain times depending on the number of layers from the master station B. In order for each of the repeater and slave stations R, T to be able to automatically recognize its number of layers from the master station B, the master station B transmits a down frame signal with a repeater number, and the repeater number is incremented by "1" in each of the repeater and slave stations R, T.
Each of the repeater stations of the proposed time-division multiplex communication system has a loop-back function to allow slave stations to communicate with each other in the event of a failure of a repeater station. If a repeater station R.sub.1 in the first layer of a time-division multiplex communication system shown in FIG. 2 fails to operate, a repeater station R.sub.2 in the second layer loses frame synchronization or is in frame asynchronization because it cannot receive a down frame signal from the repeater station R.sub.1. At this time, the repeater station R.sub.2 performs its loop-back function to generate a substitute down frame signal by itself, and transmits signals, which are composed of the substitute down frame signal with up burst signals from slave stations T.sub.1 -T.sub.3, to the slave stations T.sub.1 -T.sub.3. As a result, there is established a network with the repeater station R.sub.2 acting as a virtual master station, allowing the slave stations T.sub.1 -T.sub.3 to communicate with each other.
The early time-division multiplex communication system is however disadvantageous in that its hardware arrangement is relatively large in scale due to a delay circuit required by each of the repeater stations to delay an up burst signal from a slave station into timed relationship to its own substitute down frame signal when they are added to each other by the loop-back function.